The present invention relates to a rotational driving apparatus with frictional engagement in which at least one frictional roller and a driving roller are pressed against a rotating member so that rotation of the driving roller is transmitted to the rotating member without causing a slip, and to a robot comprising a driving apparatus using the rotational driving apparatus with frictional engagement.
As a conventional rotational driving apparatus used in, e.g., an articulated robot, a technique of transmitting a rotational force from a driving motor through a reduction gear is known, as disclosed in Japanese Patent Laid-Open (Kokai) No. 60-161083 (corresponding to U.S. Pat. No. 4,630,496). However, in the technique of transmitting the driving force through the gear, a backlash inherent to a gear or wear of a gear tooth flank occurs, thus posing problems of rotational positioning precision and durability.
For this reason, as a rotational driving apparatus without a gear, a planet roller type power transmission apparatus for transmitting power by a frictional force of rollers contacting each other is known, as disclosed in, e.g., U.S. Pat. No. 4,483,216. This apparatus has the following construction. That is, a thick portion is formed in an outer circumferential portion to locate a neutral surface near the outer circumference, thereby increasing a deformation property of a contact portion with another roller. Thus, this conventional apparatus accomplishes the object in which a large pressing force is obtained by a small axial urging force. In addition, a mechanical compression means is used without a hydraulic system, thus achieving a simple structure and low cost.
In order to achieve the above object, the conventional planet roller type power transmission apparatus comprises an elastic roller, so that a frictional force is generated between a plurality of planet rollers and a sun roller based on elastic deformation of the elastic roller.
Furthermore, Japanese Patent Publication No. 58-54948 discloses a feeding apparatus in which a plurality of driving rollers are arranged so that frictional surfaces of frictional plates are pressed against a moving member from both sides, and at least one of these driving rollers is coupled to and driven by a driving apparatus.
However, in the conventional planet roller type power transmission apparatus, since a plurality of planet rollers are in rolling contact with a sun roller, if the diameters of the plurality of planet rollers are different from each other, they cannot be essentially in rolling contact with the sun roller. As a result, rotational transmission is unbalanced. The planet rollers having larger diameters are extremely worn. In this manner, the diameters of these planet rollers must have high dimensional precision. As a result, in the planet roller type power transmission apparatus, a problem of an increase in cost is posed.
Since a deformation amount of the elastic roller is very small, if the elastic roller is worn during use for a long period of time, a slip occurs between the elastic roller and the planet rollers. In this case, when a slip occurs between the elastic roller and the planet rollers, a transmission torque is decreased. For this reason, when a slip occurs, an elastic deformation amount of the elastic roller must be adjusted again, thus posing a problem of maintenance. In addition, there is no means for reliably predicting a time of a slip, and the elastic deformation amount must be periodically adjusted, resulting in a cumbersome operation.
A rotational driving apparatus with frictional engagement is suitable for applications in a rotation transmission apparatus of a robot, in particular, a vertical articulated or scalar articulated robot. Such an articulated robot has a large load weight since it is used for gripping and conveying heavy articles. For this reason, in rotational transmission by the elastic roller, the elastic deformation amount of the elastic roller is varied by the load weight. In this manner, when the elastic deformation amount is varied, it is difficult to assure precision of movement control of movable portions in the rotational driving apparatus.